The configuration and electronic state of SO3 adsorbed on Au surface

We evaluated the adsorption of SO₃ molecule on Au (1 1 1) surface using first principles calculation by a slab model with a periodic boundary condition. We find that there are six stable adsorption configurations on an Au surface, where the SO₃ molecule is adsorbed above the three-fold fcc and hcp hollow sites and on the atop site. In two of these configurations, S and two O atoms are bound to the Au atoms, the next two configurations have all the three O atoms bound to the Au surface atoms, and the last two configurations have the S atom bound to an Au surface atom on the atop site and O atoms situated above the hollow sites. In these configurations, the electronic structures of SO₃ on the Au surface show that molecular orbitals of SO₃ and those of the Au surface are hybridized in the active metal d-band region, that the localized molecular orbitals in SO₃ are stabilized, and that charge is transferred from Au to S 3p by SO₃ adsorption on the Au surface though there is little other interaction of the S and O (bound to Au) component with Au. Moreover, the bond between the S and O atoms bound to Au is weakened due to SO₃ adsorption on the Au surface due to the charge polarization of the O-Au bond. This interaction is likely to encourage the S-O bond to break.     

Pressure effect on the electronic and optical properties of the alkali antimonide semiconductors Cs3Sb, KCs2Sb, CsK2Sb and K3Sb: Ab initio study

We present first principles calculations of the effect of pressure on the electronic and optical properties of the alkali antimonides semiconductors K₃Sb, K₂CsSb, KCs₂Sb and Cs₃Sb by means of the full-potential linearized augmented plane wave method within the generalized gradient approximation. The band gap variation is not linear. The crossover pressure values are determined for K₃Sb and K₂CsSb. Under pressure the structures in the optical spectra shift towards higher energies for K₃Sb and KCs₂Sb whereas the threshold energy is lowered for K₂CsSb and Cs₃Sb. The electronic dielectric constant decreases with pressure for K₃Sb while it increases for the other three compounds. Our results indicate that the absorption becomes strong in the UV region for KCs₂Sb and Cs₃Sb.     

Enhanced gas-sensing behaviour of Ru-doped SnO2 surface: A periodic density functional approach

A theoretical study on Ru-doped rutile SnO₂(1 1 0) surface has been carried out by means of periodic density functional theory (DFT) at generalized gradient approximation (GGA-RPBE) level with a periodic supercell approach. Electronic structure analysis was performed based on the band structure and partial density of states. The results provide evidence that the electronic structures of SnO₂(1 1 0) surface are modified by the surface Ru dopant, in which Ru 4d orbital are located at the edge of the band gap region. It is demonstrated that molecular oxygen adsorption characteristics on stoichiometric SnO₂(1 1 0) surface are changed from endothermic to exothermic due to the existence of surface Ru dopant. The dissociative adsorption of molecular oxygen on the Ru_5c/SnO₂(1 1 0) surface is exothermic, which indicates that Ru could act as an active site to increase the oxygen atom species on SnO₂(1 1 0) surface. Our present study reveals that the Ru dopant on surface is playing both electronic and chemical role in promoting the SnO₂ gas-sensing property.     

Crystal growth and the electronic structure of Tl3PbCl5

Total and partial densities of states of constituent atoms of two tetragonal phases of Tl₃PbCl₅ (space groups P4₁2₁2 and P4₁) have been calculated using the full potential linearized augmented plane wave (FP-LAPW) method and Korringa-Kohn-Rostoker method within coherent potential approximation (KKR-CPA). The results obtained reveal the similarity of occupations of the valence band and the conduction band in the both tetragonal phases of Tl₃PbCl₅. The FP-LAPW and KKR-CPA data indicate that the valence band of Tl₃PbCl₅ is dominated by contributions of the Cl 3p-like states, which contribute mainly to the top and the central portion of the valence band with also significant contributions throughout the whole valence-band region. Further, the bottom of the valence band of Tl₃PbCl₅ is composed mainly of the Tl 6s-like states, while the bottom of the conduction band is dominated by contributions of the empty Pb 6p-like states. The KKR-CPA results allow to assume that the width of the valence band increases somewhat while band gap, E_g, decreases when changing the crystal structure from P4₁2₁2 to P4₁. The X-ray photoelectron core-level and valence-band spectra for pristine and Ar⁺-ion-irradiated surfaces of a Tl₃PbCl₅ monocrystal grown by the Bridgman-Stockbarger method have been measured.     

Electronic structure and chemical bonding of phosphorus-contained sulfides InPS4, TI3PS4, and Sn2P2S6

The electronic structure of phosphorus-contained sulfides InPS₄, Tl₃PS₄, and Sn₂P₂S₆ was investigated experimentally with X-ray spectroscopy and theoretically by quantum mechanical calculations. The partial densities of electron states calculated with the ab initio multiple scattering FEFF8 code correspond well to their experimental analogues—the X-ray K- and L_2,3-spectra of sulfur and phosphorus. The good agreement between theory and experiment was also achieved for K-absorption spectra of S and P in the investigated sulfides. In spite of the difference in the crystallographic structure of InPS₄, TI₃PS₄, and Sn₂P₂S₆ that influence the form of K-absorption spectra, the electronic structure of their valence bands are rather similar. This is due to the strong interaction of the P and S atoms, which are the nearest neighbors in the compounds studied. The electron densities of p- and s-states of phosphorus are shifted by about 3 eV to lower energies in comparison to the analogous electron states of sulfur. This is connected with the greater electro-negativity of sulfur, and is confirmed by the calculated electron charge transfer from P to S.     

The effects of O deficiency on the electronic structure of rutile TiO2

Ab initio density functional calculations (plane wave GGA, CASTEP) were performed to determine the effect of O deficiency on the electronic structure of rutile, TiO₂. O deficiency was introduced through either the removal of O or the insertion of interstitial Ti atoms. At physically realistic concentrations of O vacancies in the rutile lattice (i.e. 25% and less) O deficiency results in the population of the bottom of the conduction band, the location of the Ti 3d orbitals in the pure structure, increasingly with increasing vacancy concentration. We propose that this could be confused with the formation and population of gap states especially where O vacancies occur in isolated positions in the lattice. In contrast, Ti interstitials introduce a defect state into the energy gap, without an overall reduction in the size of the energy gap. O vacancies result in a spin polarized solution, whereas Ti interstitials do not.     

Doping-induced spin-manipulation in complex trimer system Ca3Cu3(PO4)4: A density functional investigation

Doping induced spin-manipulation with magnetic (Ni) and non-magnetic (Mg) dopants constitutes the experimental attempts to obtain a singlet ground state system from the linear chain Heisenberg antiferromagnetic Cu-based d⁹ spin-1/2 trimer compound Ca³Cu³(PO⁴)⁴ with doublet ground state. The present study is a density-functional investigation of the effects of such doping on the spin-exchange mechanism and electronic structure of the parent compound. Site-selective doping with zero-spin dopants like Mg is proved to be more efficient than an integral spin dopant Ni in obtaining a spin-gap system with singlet ground state, as also observed in the experimental studies. Doping induced dimerized state is found to be the lowest in ground-state energy. Calculated spin exchange couplings along various possible pathways are observed to attain good agreement with earlier experimental results with suitable optimization of Coulomb repulsion (U) and exchange (J) parameters.     

Structural and electronic properties of pyrrolidine-functionalized [60]fullerenes

We have investigated energetic, geometric, electronic, and field emission properties of three recently synthesized fulleropyrrolidines based on the density functional theory method B3LYP/6-31G(d). Fulleropyrrolidines show higher conductivity, and solubility in water, and smaller work function in comparison with the pristine C₆₀ fullerene. The functionalization of C₆₀ with different pyrrolidines containing 3NH₂, NO, or NO₂ groups transforms it to an n-type semiconductor. The functionalization can also dramatically enhance the electrophilicity of the C₆₀ about 23–37%. Moreover, it should be mentioned that the work function is mainly influenced by the pyrrolidine containing 3NO₂ group whereas the conductivity is largely affected by the one containing 3NH₂ functionality.     

Preparation of Fe2O3/SrTiO3 composite powders and their photocatalytic properties

Fe₂O₃/SrTiO₃ composite powders have been prepared and their photocatalytic activities were investigated by photooxidizing methanol. These powders were characterized by ultraviolet (UV)-visible diffuse reflectance spectra, scanning electron microscope (SEM) and X-ray diffraction (XRD). The results showed that the Fe₂O₃/SrTiO₃ composite powders with optimum proportion exhibited higher photocatalytic activity than pure SrTiO₃, Fe₂O₃ and TiO₂ (P25) under visible light (λ>440 nm) irradiation. The SEM image of the composite powders showed that SrTiO₃ and Fe₂O₃ particles contacted well. Further research revealed that the calcination temperature is an important factor in the preparation of the composite powder with relatively high photocatalytic ability.     

Comparative structural and photoluminescent study of Eu-doped La2O3 and La(OH)3 nanocrystalline powders

Eu³⁺-doped La₂O₃ nanocrystalline powder was prepared by polymer complex solution method and further used for preparation of Eu³⁺-doped La(OH)₃. Structural and optical characterization was carried out by powder X-ray diffraction and photoluminescent spectroscopy. XRD measurements confirmed the formation of hexagonal La₂O₃ and its recrystallization into La(OH)₃ in a humid atmosphere. Excitation spectra show redshift of host lattice and charge transfer emission bands in La(OH)₃ while bands that correspond to Eu³⁺f–f transitions are placed at same wavelengths in both samples. Photoluminescence spectra recorded over the temperature range from 10 K to 300 K show that intensities of emission lines in Eu³⁺-doped La₂O₃ do not depend on temperature as much as in La(OH)₃ sample. Observed dominant ⁵D₀→⁷F₂ and markedly visible ⁵D₀→⁷F₀ emissions in doped La₂O₃ indicate that Eu³⁺ ion is located in a structural site without an inversion center. On the other hand, in Eu³⁺-doped La(OH)₃⁵D₀→⁷F₀ transition is barely visible while ⁵D₀→⁷F₂ is not prominent, and with temperature drop three ⁵D₀→⁷F_J (J=1, 2, 4) transitions become almost of the same intensity. In both La₂O₃ and La(OH)₃ structures Eu³⁺ ion replaces La³⁺ in non-centrosymmetric C_3v and C_3h crystallographic sites, respectively, and difference in symmetry of the crystal field around europium ion is explained by comparing shape and volume of these sites. Decay times of the ⁵D₀- level recorded over the temperature range 10−300 K revealed that emission lifetime values in La₂O₃ (~0.7 ms) are almost two times higher than in La(OH)₃ (~0.4 ms), and unlike in La₂O₃, lifetime in La(OH)₃ is temperature dependent.     

First-principles calculations of electronic, optical and elastic properties of ZnAl2S4 and ZnGa2O4

The optimized crystal structures, band structures, partial and total densities of states (DOS), dielectric functions, refractive indexes and elastic constants for ZnAl₂S₄ and ZnGa₂O₄ were calculated using the CASTEP module of Materials Studio package. Pressure effects were modeled by performing these calculations for different values of external hydrostatic pressure up to 50 GPa. Obtained dependencies of the unit cell volume on pressure were fitted by the Murnaghan equation of state, and the relative changes of different chemical bond lengths were approximated by quadratic functions of pressure. Variations of applied pressure were shown to produce considerable re-distribution of the electron densities around ions in both crystals, which is evidenced in different trends for the effective Mulliken charges of the constituting ions and changes of contour plots of the charge densities. The longitudinal and transverse sound velocities and Debye temperatures for both compounds were also estimated using the calculated elastic constants.     

The electronic and structural properties of BN and BP nano-cages interacting with OCN: A DFT study

The adsorption of OCN⁻ (cyanato anion) on boron nitride (B₁₂N₁₂ and B₁₆N₁₆) and boron phosphide nano-cages (B₁₂P₁₂ and B₁₆P₁₆) in terms of energetic, geometric, and electronic properties are studied using density functional theory calculations. Our study results indicated that the first OCN⁻ strongly prefers to be adsorbed from its N atom upon B atoms of the nano-cages than the O atoms of OCN⁻. These findings have been rationalized using frontier molecular orbitals and total electron density plots. The energy gap of the B₁₂P₁₂ is significantly reduced upon the adsorption of OCN⁻ compared to B₁₂N₁₂, thus leading to the increase in electrical conductance of nano-cage.     

Analysis of electronic structures of 3d transition metal-doped TiO2 based on band calculations

The electronic structures of titanium dioxide (TiO₂) doped with 3d transition metals (V, Cr, Mn, Fe, Co and Ni) have been analyzed by ab initio band calculations based on the density functional theory with the full-potential linearized-augmented-plane-wave method. When TiO₂ is doped with V, Cr, Mn, Fe, or Co, an electron occupied level occurs and the electrons are localized around each dopant. As the atomic number of the dopant increases the localized level shifts to lower energy. The energy of the localized level due to Co is sufficiently low to lie at the top of the valence band while the other metals produce midgap states. In contrast, the electrons from the Ni dopant are somewhat delocalized, thus significantly contributing to the formation of the valence band with the O p and Ti 3d electrons. Based on a comparison with the absorption and photoconductivity data previously reported, we show that the t_2g state of the dopant plays a significant role in the photoresponse of TiO₂ under visible light irradiation.     

O-vacancy and surface on CeO2: A first-principles study

Atomic and electronic structures of CeO₂ (1 1 1), (1 1 0) and (1 0 0) surfaces are investigated using the first-principles density functional theory taking into account the on-site Coulomb interaction. Both the stoichiometric and O-deficient surfaces are examined in order to clarify the overall features. The CeO₂ (1 1 1) is found to be the most stable surface, followed by the (1 1 0) and (1 0 0) surfaces, consistent with experimental observations. Three surfaces exhibit different features of relaxation. Large relaxations are found at the (1 1 0) and (1 0 0) surfaces, while very small changes are observed at the (1 1 1) surface. It is found that the O-vacancy occurs more readily at the (1 1 0) surface as compared with the (1 1 1) surface. Furthermore, the formation energies of the O-vacancy in the surfaces are lower than that in the bulk. The energetically favorable O-vacancy locates in the second O-atomic layer for the (1 1 1) while at the surface layer for the (1 1 0). The excess electrons left with the removal of the O atom are distributed in the first two layers with certain (a considerable) fraction filling the Ce-4f states.     

Structural and vibrational properties of Ca2FeH6 and Sr2RuH6

The structural and vibrational properties of the isostructural compounds Ca₂FeH₆ and Sr₂RuH₆ are determined by periodic DFT calculations and compared with their previously published experimental crystal structures as well as new experimental vibrational data. The analysis of the vibrational data is extended to the whole series of alkaline-earth iron and ruthenium hydrides A₂TH₆ (A=Mg, Ca, Sr; T=Fe, Ru) in order to identify correlations between selected frequencies and the T-H bond length. The bulk moduli of Ca₂FeH₆ and Sr₂RuH₆ have also been determined within DFT. Their calculated values prove to compare well with the experimental values reported for Mg₂FeH₆ and several other compounds of this structure.     

Lattice dynamics of cubic SrZrO3

Using density functional perturbation theory, the optical dielectric constant, Born effective charges and phonon dispersion curves of cubic SrZrO₃ have been calculated. The obtained dispersion curves show a soft phonon branch spreading from R to M points of the cubic Brillouin zone. An analysis based on the symmetry relationships indicates that the experimentally observed low-symmetry phases of SrZrO₃ can be considered as results of the soft mode condensation at R and M points.     

Molecular dynamics simulation of phase transition in AgNO3

Structural phase transition in AgNO₃ at high temperature is simulated by molecular dynamics. The simulations are based on the potentials calculated from the Gordon-Kim modified electron-gas formalism extended to molecular ionic crystals. AgNO₃ transforms into rhombohedral structure at high temperature and the phase transition is associated with the rotations of the NO₃ ions and displacements of the NO₃ and Ag ions.     

Intense and broad photoluminescence at room temperature in structurally disordered Ba[Zr0.25Ti0.75]O3 powders: An experimental/theoretical correlation

Intense and broad photoluminescence (PL) emission at room temperature was observed on structurally disordered Ba[Zr_0.25Ti_0.75]O₃ (BZT) powders synthesized by the polymeric precursor method. BZT powders were annealed at 573 K for different times and at 973 K for 2 h in oxygen atmosphere. The single-phase cubic perovskite structure of the powder annealed at 973 K for 2 h was identified by X-ray diffraction and Fourier transform Raman techniques. PL emission increased with the increase of annealing time, which reached its maximum value in the powder annealed at 573 K for 192 h. First principles quantum mechanical calculations based on density functional theory (B3LYP level) were employed to study the electronic structure of ordered and disordered models. The theoretical calculations and experimental measurements of Ultraviolet-visible absorption spectroscopy indicate that the presence of intermediary energy levels in the band gap is favorable for the intense and broad PL emission at room temperature in disordered BZT powders. The PL behavior is probably due the existence of a charge gradient on the disordered structure, denoted by means of a charge transfer process from [TiO₅]-[ZrO₆] or [TiO₆]-[ZrO₅] clusters to [TiO₆]-[ZrO₆] clusters.     

Theoretical study of HOCl adsorption on ice surface

Adsorption of HOCl on ice surface was studied using the ab initio molecular orbtial theory. We applied Hartree–Fock (HF) self-consistent field and the second-order Møller–Plesset (MP2) level of theory to cluster models of the (0001) surface ice Ih to optimize adsorption structures and binding energies. In all stable binding configurations, HOCl acts as a proton donor in a hydrogen bond. The presence of neighboring water molecules can strengthen the interaction of HOCl with ice. In the HOCl·(H₂O)₄ system, interaction hydrogen bond length is about 1.85 Å, and binding energies are −10.063−11.149 kcal mol⁻¹. We also calculated the vibrational frequencies of HOCl affected by the ice surface.